Performance of a front-end preamplifier with a digital output for semiconductor detectors

In several areas, it is necessary to digitise the output of a radiation detector for subsequent processing. Examples of such systems include amorphous silicon active matrix image arrays and CdZnTe spectrometers. This paper discusses a preamplifier that is designed to satisfy such applications. The buffered and inverted outputs from the detector are fed via a multiplexor to a current integrator whose output is monitored by two comparators. Using a J-K flip-flop, the comparator outputs control the direction of current passed to the integrator. The result is a frequency modulated logic pulse train that can be counted to determine the integral detector signal. The minimum time between pulses gives the peak slew-rate of the detector signal. An analysis of the circuit is presented to indicate the constraints on its performance.